This invention relates to analog-to-digital converters (ADCs).
Continuous time Bandpass Delta Sigma (xcex94xcexa3) modulators are used in ADC systems for digitizing signals of wide dynamic range, e.g., 14 to 16 effective bits in a wide information bandwidth, e.g. 60-100 MHz centering at an IF frequency. Bandpass Delta Sigma modulators typically include a loop filter (also called a resonator for the bandpass modulator), a single bit or multi-bit quantizer, and a single bit or multi-bit feedback digital-to-analog converter (DAC). The loop filter structure can be a sampled-data discrete time filter or a continuous time filter. The discrete time filter is typically implemented in CMOS switched-capacitor technology that has relatively low signal bandwidth. The continuous time loop filter can be implemented in advanced bipolar technology that allows much higher amplifier bandwidth, faster settling and permits the complete modulator to sample at a higher clock rate. This in turn leads to wider signal bandwidth and superior dynamic performance.
A basic second order continuous time bandpass xcex94xcexa3 modulator ADC topology is described in xe2x80x9cA 3.2 GHz Second-Order Delta-Sigma Modulator Implemented in InP HBT Technology,xe2x80x9d Jensen et al, IEEE J. Solid State Circuits, October 1995. The topology has a cascade of two integrators in series in a resonator configuration that drives a one bit comparator-DAC feedback. This topology does not have any compensation for delay in the loop. To achieve greater ADC dynamic range, this topology can be extended into a higher order xcex94xcexa3 modulator loop by cascading more of the same second order resonator loops. However this topology suffers from additional unnecessary delay in the signal path because of having all integrators cascading together in series, i.e., a 6th order modulator would have 6 integrators in series. Excess delay impacts stability in a high order modulator structure.
A feedforward signal compensation method is described in xe2x80x9cAn Eighth-Order Bandpass xcex94xcexa3 Modulator for A/D Conversion in Digital Radio,xe2x80x9d Louis et al, IEEE J. Solid State Circuits, April 1999. The compensation method is applied to a sampled-data resonator (not a continuous time resonator) structure with all switched-capacitor integrators configured in cascade. This sampled data resonator structure has limited frequency bandwidth and is not preferred for digitizing wideband signal at greater than 100 MHz IF frequencies in today""s process technologies. This structure also suffers from similar delay issues as discussed above.
A continuous time Bandpass Delta Sigma (xcex94xcexa3) Modulator architecture with feedforward and feedback coefficients to completely specify both the signal transfer function (STF) and the noise transfer function (NTF) for a stable modulator ADC system is described. In an exemplary embodiment, the structure is capable of implementing any desirable noise shaping response for maximum dynamic range and producing optimal signal transmission response.
In an exemplary embodiment, the new architecture introduces an optimal number of feedforward elements to completely specify the frequency response of the modulator""s signal transfer function (STF), hence producing any specified gain flatness and/or phase linearity. Without STF optimization from the feedforward technique, a general modulator topology suffers from much undesired gain variation, e.g.,  greater than 10 dB may be observed across the signal bandwidth of interest, resulting in necessary gain correction in the digital signal processing (DSP) backend. A compensation technique in accordance with an aspect of the invention corrects for the inherent error in a general xcex94xcexa3 architecture and potentially reduces overall system complexity in subsequent DSP hardware.
In an exemplary embodiment, the topology configures the second integrator in a second order modulator in a local feedback such that only half as many integrators are in the forward signal path. An excess delay compensation technique can be integrated with the feedforward compensation technique.